A key component of a cellular phone using the Global System for Mobile Communications (GSM) is the Power Amplifier Module (PAM). Ruggedness of the PAM is determined by load mismatches that the PAM can survive under battery and temperature extremes. Under these extreme conditions, the output stage of the PAM is severely stressed.
The output stage consists of several transistors in parallel in order to meet the power requirements of the GSM phone. As the PAM is stressed, small thermal differences in the emitter paths between the parallel transistors in the output stage can lead to unmatched base-emitter voltages (Vbe). In the extreme environment of high battery voltage and load mismatches where the currents are significantly high, the small differences in Vbe can lead to thermal runaway and the self destruction of the output stage of the PAM. Accordingly, it is desirable to provide a PAM that reduces the thermal differences in the output stage and the effects of load mismatches.